1. Field of the Invention
This invention relates to a commercial process for the preparation of 2,5-hexanedione, i.e., acetonylacetone, from allylacetone and more particularly relates to the oxidation of allylacetone using palladium chloride as the catalyst in the presence of copper chloride and oxygen to prepare high yields of 2,5-hexanedione with only small losses of the palladium catalyst.
2. Description of the Prior Art
2,5-Hexanedione or acetonylacetone is important as an organic chemical intermediate. Preparation of this compound through various synthesis routes has been reported in the prior art. For example, Adams et al. in J. Am. Chem. Soc., Vol. 72, p 4368 (1950) describe the synthesis of 2,5-hexanedione by condensing propylene oxide with acetoacetic acid esters to produce alpha-aceto-gamma-valerolactone, which, in turn, is reacted with dilute hydrochloric acid and converted into 5-hydroxy-2-hexanone. To obtain 2,5-hexanedione, the hydroxy-hexanedione product is then oxidized together with sodium dichromate and sulfuric acid. Also, Shenk in Ber., Vol. 77, p 661 (1944) describes the preparation of 2,5-hexanedione by oxidizing 2,5-dimethylfuran to 3-hexene-2,5-dione, which product then is hydrogenated to produce 2,5-hexanedione. Still further, in U.S. Pat. No. 2,525,672, Heilbron et al. describe the preparation of 2,5-hexanedione by first reacting 1-bromo-2,3-epoxybutane with monosodium acetylide in liquid ammonia, and then reacting the 3-hexene-5-yn-2-ol product obtained with mercury sulfate and sulfuric acid.
For one or more reasons, however, all of these prior art processes are disadvantageous for preparing 2,5-hexanedione conveniently and economically. The process of Adams et al is a multistage process which provides only low yields of product. The Shenk process, besides providing only moderate yields of product, utilizes 2,5-dimethylfuran which is obtained only with difficulty. The Heilbron et al process likewise utilizes reactants which are difficult to handle and only moderate product yields are realized.
More recently, in Kogyo Kaguku Zasshi 71, (6), p 945-6 (1968) as well as in Japanese Patent Publication No. 1972-11411, Takamori Konaka and Sadao Yamamoto have described a simplified, one-step process for producing good commercial yields of 2,5-hexanedione from allylacetone in a mixed solvent system composed of water and either benzene or dimethylformamide. In this process, palladium chloride is employed as the oxidation catalyst in the presence of prescribed amounts of cupric chloride and oxygen. The process is carried out at temperatures ranging usually from 60.degree.-80.degree. C for an overall time period ranging generally from 3 to 12 hours but typically from 7 to 12 hours. Upon completion of the reaction, the 2,5-hexanedione product is reported to be easily recoverable from the reaction mixture and purified. However, from practice of this process, substantial quantities of undesirable byproducts are formed and losses of the expensive palladium chloride catalyst component are found to be substantial.
It has now been found that by conducting the oxidation process in a manner similar to that described by Konaka and Yamamoto, but with the use of much greater copper/palladium mole ratios and copper/allylacetone mole ratios than previously employed, commercially attractive yields, i.e., greater than about 70%, by weight of theoretical, of 2,5-hexanedione product can be obtained in significantly shorter reaction times. Most important, practice of this process results in much reduced losses of the expensive palladium catalyst.